Conventional devices of the type here envisaged comprise an ancillary resonant cavity connected via a coupling aperture or iris with the anode cavity and provided with a partly magnetized ferrite insert whose magnetization may be increased or diminished by the application of a variable control current thereto. The ancillary resonant cavity may be designed as a dual-mode phase shifter in the shape of a waveguide of square or circular cross-section short-circuited at one end, the current-controlled ferrite insert being preceded by a similar insert of constant magnetization serving to transform a linearly polarized incoming microwave in a nonreciprocable manner into a circularly polarized wave whose phase velocity is modified by the other insert. With such a tuning device, giving rise to a standing wave of circular polarization whose wavelength depends on the degree of magnetization of the second ferrite insert and thus on the intensity of the control current, the operating frequency of the magnetron may be modified within a range of roughly 1%.
Within that range of adjustment, however, the operating efficiency of the tuning device decreases significantly from the midfrequency toward the limits of the range whereby pulses generated in the output of the magnetron have amplitudes which vary undesirably with the tuning frequency. Moreover, such a known tuning device cannot be used with high-power magnetrons since the waveguide serving as an extension of the anode cavity must be limited in its transverse dimensions in order to avoid the excitation of higher modes. Thus, the ferrite inserts present in that waveguide are subjected to relatively strong magnetic microwave fields which cannot be increased at will if the generation of unstable spin waves is to be avoided.